The present invention relates to a leakage detecting method and, more particularly, to a leakage detecting method for use in an oxidizing system of forming oxide layers. The present invention also relates to a method for estimating a thickness of an oxide layer formed on a wafer.
The growth of a silicon dioxide layer is very important for achieving high quality of the integrated circuits. Generally, two processes are widely used form silicon dioxide layers. One process is employed to form native oxide layers, and the other process is used to form thermal oxide layers.
The process for forming a native oxide layer is performed by exposing a silicon wafer to an oxygen-containing atmosphere such as oxygen gas or steam at about room temperature. The native oxide layer grows very slowly and generally has a thickness of about 10 to 20 Å. The mechanism for forming a native oxide layer can be illustrated as one of the following reactions:Si(s)+O2(g)→SiO2(s)  (1)Si(s)+2H2O(g)→SiO2(s)+2H2(g)  (2)
The formulae (1) and (2) are usually referred to as dry oxidation and wet oxidation, respectively. Under some circumstances, native oxide layers are useful for enhancing interface properties between the surface of a semiconductor substrate and an insulator layer so as to provide high-quality electrical insulators for electrical isolation of a semiconductor device. Under some circumstances, due to many polar groups carried by the native oxide layers, some organic molecules having polar groups might be bonded to native oxide layers via hydrogen bonds or hydrophobic bonds. As the thicknesses of the native oxide layers increase to some extent, their interface properties will decrease and their irregular morphology might lead to the increase of surface roughness. Therefore, the growth of the native oxide layer needs to be stopped when the thickness reaches about 20 Å.
The process for forming a thermal oxide layer is performed at a temperature ranging from about 700° C. to about 1200° C. by dry oxidation or wet oxidation as described above. Depending on applications of the semiconductor devices, the thermal oxide layer generally has a thickness of about 300 to about 20,000 Å. The thermal oxide layers are suitable for use in forming, for example, a field oxide layer, a dielectric layer, a gate oxide layer, and the like.
FIG. 1 is a schematic view illustrating a typical oxidizing system for forming an oxide layer on a wafer by means of a wet oxidation process. Such an oxidizing system comprises a quartz furnace tube 11, a torch 12, a steam chamber 13 and a piping system. The piping system principally comprises some feeding pipes for introducing reacting gases and purge gas, and an exhaust pipe (not shown) for discharging exhaust gases. The reacting gases generally comprise hydrogen gas (H2) and oxygen gas (O2) for a wet oxidation process. Alternatively, for a dry oxidation process, only oxygen gas is required for forming oxide layers. The purge gas such as nitrogen (N2) is employed to purge the whole oxidizing system prior to feeding the reacting gases. The valves V1, V2 and V3 in the feeding pipes are used to control the open/close states or flow rates of the nitrogen gas, hydrogen gas and oxygen gas, respectively. The exhaust pipe is usually installed from a top vent of the quartz furnace tube 11 for discharging the exhaust gas after the oxidation.
FIG. 2 (including FIGS. 2A and 2B) is a flowchart illustrating a process for forming an oxide layer on a wafer according to prior art. Such process comprises two main procedures, i.e., a preliminary procedure and a normal oxidizing procedure. The preliminary procedure is performed for ascertaining whether there is a leakage of the overall oxidizing system. The preliminary procedure is described as follows and is shown with reference to FIGS. 1 and 2A. First, a plurality of test wafers are placed into the quartz furnace tube 11 which serves as an oxidizing chamber (Step S11). Then, the valves V2 and V3 are closed, and the valve V1 is kept open (Step S12). In Step S13, nitrogen is introduced into the oxidizing system at a specified flow rate so as to purge the overall oxidizing system for about 5 minutes. Then, the quartz furnace tube 11 is heated to an operating temperature for forming oxide layers on the test wafers, e.g., 800 to 1000° C., and maintained at such temperature for about 10 to 20 minutes (Step S14). Then, the temperature of the quartz furnace tube 11 is decreased to about room temperature for about 1 to 1.5 hours (Step S15). Then, as shown in Step S16, the test wafers are removed from the quartz furnace tube 11, and an average thickness d of the oxide layers on the test wafers is measured. If the average thickness d is greater than an acceptable thickness, e.g., 20 Å, it indicates that some leakages might be generated in the piping system or associated connectors. Meanwhile, the actual locations of leakages need to be detected. In addition, some remedial measures should be taken to prevent leakage, such as re-tightening the connectors and/or welding the pipes. To assure that there is no additional leakage in the oxidizing system, the steps S11 to S16 should be repeated.
Alternatively, if the average thickness d is less than the acceptable thickness, it means that no leakage occurs in the oxidizing system. At that time, the normal oxidizing procedure will be performed, as described below with reference to FIG. 2B.
First, a batch of working wafers for forming thereon oxide layers are placed into the quartz furnace tube 11 (Step S21). Then, the valves V2 and V3 are closed, and the valve V1 is kept open (Step S22), and nitrogen is subsequently introduced into the oxidizing system at a specified flow rate so as to purge the overall oxidizing system for about 5 minutes (Step S23). Then, the quartz furnace tube 11 is heated to an operating temperature for forming oxide layers on the working wafers, e.g., 800 to 1000° C., and maintained at such temperature for about 10 to 20 minutes (Step S24). Then, the valve V1 is closed, and the valves V2 and V3 are opened in order to introduce oxygen and hydrogen gases into the torch 12 (Step S25). The oxygen and hydrogen gases combined in the torch 12 will ignite and burn to produce steam. The steam enters the steam chamber 13 and the quartz furnace tube 11. Depending on the application, the processing time is controlled to form a desirable thickness for each oxide layer on the working wafers (Step S26).
The above-mentioned preliminary procedure has some drawbacks. For example, the preliminary procedure is time-consuming for detecting leakage of the overall oxidizing system, because the average oxide layer thickness of the test wafers is obtained after the quartz furnace tube 11 has been cooled down. In addition, if some leakages are likely to be generated, it is necessary to re-tighten the connectors and/or weld the pipes and carry out the above preliminary procedure again until no leakage is detected. That is to say, when some leakages occur, the preliminary procedure will be carried out at least two times. Since every preliminary procedure takes about 1.5-2 hours, it takes a relatively long time period, e.g., approximately 0.5 to 1.0 day, and wastes a substantial number of test wafers for testing the leaking condition. Therefore, there is a need to develop an improved process for detecting a leaking condition of the oxidizing system so as to overcome the above-mentioned problems.